This invention relates to part-turn or multi-turn rotary drives which are indexed to specific angular positions by means of a detent mechanism, such as a spring-loaded roller and cam mechanism or its mechanical or magnetic equivalent, and to means for electrically driving such mechanisms. An example of such a drive is a rotary selector switch, which may typically have four, six, eight or twelve switch detent positions at 90.degree., 60.degree., 45.degree., or 30.degree. indexing angles.
Where such devices are normally operated manually, the manual effort required is to overcome the detent spring force to move from one index position towards the next; as soon as the half-way position is passed, the detent spring is no longer in opposition, but applies torque to bring the drive to the next detent position.
However, if an electric motor is used to drive such a mechanism, the combination of the motor torque and inertia and the forward driving torque of the spring tends to cause overshooting of the next position, leading to unsatisfactory operation.